One strategy for reducing nitrogen oxide exhaust gas emissions in an internal combustion engine involves the introduction of a reagent comprising a reducing agent, typically a liquid ammonia source such as an aqueous urea solution, into the exhaust gas stream. This method is known as selective catalytic reduction or SCR. The reducing agent is injected into the exhaust gas upstream of an exhaust gas catalyst, known as an SCR catalyst. Nitrogen oxides in the exhaust gas undergo a catalysed reduction reaction with the ammonia source on the SCR catalyst, forming gaseous nitrogen and water.
Typically, in a selective catalytic reduction (SCR) system, injection of the reagent into the exhaust gas stream is achieved by pumping the reagent from a supply tank to an injection nozzle disposed within the exhaust gas stream using a suitable pump, such as described in the present applicant's co-pending European Patent Application Publication No. EP-A-1878920.
FIG. 1 shows, schematically and in simplified form, a known pump 20 suitable for pumping reagent in an SCR system. The pump 20 comprises a solenoid actuator 22 disposed within a generally cylindrical housing 24. The actuator 22 comprises a tubular pole member 26, formed integrally with the housing 24, and a wire winding or coil 28 disposed around the pole member 26. One end of the pole member 26 forms an annular pole face 30 of the actuator 22.
An armature 32 is provided in an armature chamber 34 adjacent to the pole face 30. The armature 32 is connected to a pumping plunger 36. The plunger 36 is slidably received in a central bore 38 of a sleeve 40 disposed centrally within the pole member 26. An end face 42 of the sleeve 40 is spaced from the pole face 30, so as to form a spring chamber 44 adjacent to the pole face 30. A biasing spring 46 is partially received in the spring chamber 44 to bias the armature 32 away from the pole face 30. One end of the biasing spring 46 abuts the end face 42 of the sleeve 40, and the other end of the biasing spring 46 abuts a central region 48 of the armature 30.
The pump 20 further comprises, at an upstream end, an inlet 50, which receives fluid from a source such as a tank (not shown) and, at a downstream end, an outlet 52 that is in communication with a delivery nozzle (not shown). In use of the pump 20 in an SCR system for an internal combustion engine, the fluid is a reagent and the delivery nozzle is disposed within an exhaust pipe of the engine.
A supply passage 54 is provided within the pump 20 to convey fluid from the inlet 50 to the outlet 52. In this example, the supply passage 54 comprises an annular space 56 between the pole member 26 and the coil 28, and further comprises radial passages 58 that extend through the pole member 26 and the sleeve 40 to communicate with the bore 38 of the sleeve 40. When the pumping plunger 36 is in the position shown in FIG. 1, with the armature 32 biased away from the pole face 30, the radial passages 58 communicate with a delivery chamber 60 formed at a downstream end of the bore 38 of the sleeve 40.
Fluid flow from the delivery chamber 60 to the outlet 52 is controlled by an outlet valve 62, which is arranged to open when the pressure of fluid in the delivery chamber 60 exceeds a threshold level.
In operation, in a pumping stroke of the pump, the coil 28 is energized to generate a toroidal magnetic field around the coil 28. As a result, the armature 32 moves towards the pole face 30, against the force of the biasing spring 46, such that the downstream end of the plunger 36 interrupts fluid flow between the radial passages 58 and the delivery chamber 60. The downstream end of the plunger 36 reduces the volume of the delivery chamber 60, so that the pressure of fluid in the delivery chamber 60 increases. Once the threshold pressure is reached, the outlet valve 62 opens to cause delivery of fluid from the outlet 52 of the pump 20.
The coil 28 is then de-energized, whereupon the magnetic forces acting on the armature 32 diminish. The force of the biasing spring 46 causes the armature 32 to move away from the pole face 30, so as to increase the volume of the pumping chamber 60 and re-open fluid communication between the radial passages 58 and the delivery chamber 60. Fluid can then re-fill the delivery chamber 60, ready for the next pumping stroke.
In an SCR system, it is desirable to provide rapid, frequent injections of fluid into the exhaust pipe. For example, to ensure sufficient atomization of the fluid as it leaves the delivery nozzle, the velocity of the pumping plunger 36 must be relatively high, typically of the order of 2 meters per second. As will be appreciated from FIG. 1, the armature 32 must move through fluid within the armature chamber 34. Since the diameter of the armature 32 is relatively large, a significant quantity of fluid is displaced when the armature 32 moves. The displacement of this fluid tends to slow the movement of the armature 32 and therefore the pumping plunger 36.
To allow the armature 32, and hence the plunger 36, to move fast enough in the armature chamber 34, it is known to provide vent holes 64 in the armature 32. The vent holes 64 extend axially through the armature 32 from the face of the armature 32 nearest the pole face 30 to the opposite face, furthest from the pole face 30. During movement of the armature 32, fluid can flow through the vent holes 64 as well as around the periphery of the armature 32, thereby reducing the fluid drag on the armature 32.
It has been found that, when the coil 28 is de-energized and the armature 32 moves away from the pole face 30, the pressure in the spring chamber 44 is caused to drop rapidly. This can lead to cavitation damage to the actuator 20, caused by the collapse of cavities in the fluid in the spring chamber 44 that form as a result of the pressure drop.
Accordingly, it would be desirable to provide an armature for an actuator that overcomes or mitigates this problem.